1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus.
2. Description of Related Art
In recent years, a motor-driven power steering apparatus, which includes an electric motor in the steering system of a vehicle and in which drive power of the electric motor is added to steering force applied by a driver, has been proposed.
For example, a motor-driven power steering apparatus disclosed in JP-A-2001-315657 is controlled by a control apparatus. First, according to steering torque, vehicle speed, or the like, the control apparatus sets a target current to be supplied to the electric motor so as to control the driving of the electric motor. The control apparatus performs feedback control to set the deviation between the target current and the actual current to zero, and thus to make the set target current coincide with the actual current actually flowing through the electric motor.
The roughness of a road, the misalignment of a wheel, or the like may cause the occurrence of a shimmy phenomenon, which is a concern. Since rotational vibration of a steering wheel is transmitted to a driver during the occurrence of the shimmy phenomenon, the vibration of the steering wheel caused by the shimmy phenomenon is desirably suppressed. For this reason, the electric motor is deemed to apply assistance force to suppress vibration caused by the shimmy phenomenon. When assistance force is applied to suppress vibration other than the vibration caused by the shimmy phenomenon, steering feeling may be deteriorated, which is a concern.